Abstract: Solid-state fermentation of sugarcane leaves was performed with Aspergillus niger for the purpose of extracting secondary metabolites. The effects of inoculum size, the water content of fermentation substrate and fermentation time on the release of phenolics from sugarcane leaves were investigated. The activities of cellulase and β-glucosidase were determined during the fermentation process. The antioxidant activity of the flavonoids extracted from native and fermented sugarcane leaves was investigated. The results showed that the contents of polyphenols and flavonoids in sugarcane leaves significantly increased after fermentation, reaching 10.40 and 13.82 mg/g on a dry basis under the following conditions: inoculum size 28 mL/100 g, water content of culture medium 50%, and fermentation time 5 days, which increased by 135% and 92% as compared with those in the unfermented samples, respectively. The activity of cellulase reached its maximum at 632.17 U/g on the fourth day of fermentation and the activity of β-glucosidase peaked at 75.02 U/g on the fifth day. Positive correlations were found between the contents of phenolic compounds and the activities of cellulase and β-glucosidase. By calculating the IC50, the flavonoids extracted from fermented sugarcane leaves exhibited stronger scavenging activities against 1,1-diphenyl-2-picrylhydrazyl (DPPH) and hydroxyl radicals than did those from the unfermented samples.

Key words: sugarcane leaves; solid-state fermentation; phenolics; cellulase; β-glucosidase; antioxidant activity